Transition elements — five key properties
Transition elements fill the central d-block between Group II and Group III; the IGCSE set is period 4 — iron, copper, manganese, nickel, zinc, chromium. Five properties separate them from Group I: high density (Group I floats on water); high melting point except mercury, liquid at room temperature; coloured compounds (Group I is white or colourless); variable oxidation states (Group I is fixed at +1); and often catalytic activity (Group I has none).
Coloured compounds of transition metals
The colour comes from the transition-metal ion, so changing the anion only shifts the shade. Copper(II) salts are blue (copper(II) chloride solution green); iron(II) pale green; iron(III) orange-brown; potassium dichromate(VI) orange; potassium manganate(VII) purple. Anhydrous copper(II) sulfate is white, turning blue with water — the test for water. Iron(II) and iron(III) are the pair most confused, so match each colour to its Roman numeral.
Variable oxidation states and catalysis
Transition metals show more than one oxidation state: iron +2 or +3, copper +1 or +2, manganese +2 to +7 (it is +4 in and +7 in ). The Roman numeral gives the oxidation state (iron(III) oxide is ). They also catalyse: iron in the Haber process, manganese(IV) oxide for decomposition, nickel for hydrogenating vegetable oils. A catalyst speeds a reaction and is chemically unchanged.
Drawn from real examiner reports.
Iron(II) vs iron(III): name and colour
Work the iron charge out from the formula, then match the colour. In two make Fe +2 → iron(II) chloride, pale green; in three make Fe +3 → iron(III) chloride, orange-brown. Writing "iron chloride" with no Roman numeral, or swapping pale green and orange-brown, loses the identification mark.
W22 P41 Q2; S23 P41 Q6(a)(ii) — oxidation states in compound names wrong or missing; iron(III) confused with iron(II).
Catalyst: state that it is unchanged
The catalyst definition needs two elements: it increases the rate AND is chemically unchanged at the end. Answers giving only "speeds up the reaction" score one mark of two. "Not used up" is accepted for the second element, but "chemically unchanged" is preferred. Remember iron is the catalyst in the Haber process, not a reactant.
S23 P41 Q4(a) — students state catalysts increase rate but omit the unchanged-at-the-end clause.
Name vs formula — answer the one asked
Examiners flag giving a formula when a name is wanted, or a name when a formula is wanted. If asked to name, write "iron(III) oxide", not ; if asked for the formula, write , not the name; if asked for the oxidation state of iron in , write "+3", not "iron(III)". Read the command word before you answer.
W22 P41 general; S23 P41 general — giving a formula when a name is asked (or the reverse) in transition-metal compound questions.
Oxidation state needs the plus sign
An oxidation state is written with its sign: "+2" or "+3", never "2" or "two". The Roman numeral in a name such as iron(III) is not itself an oxidation-state answer — convert it to "+3" when the oxidation state is asked for. When naming, the Roman numeral must appear (iron(III) oxide, not iron oxide) because iron has more than one possible state.
State the colour, not just coloured
A colour question will not accept the word "coloured" — give the specific colour: copper(II) blue, iron(II) pale green, iron(III) orange-brown, potassium manganate(VII) purple. The colour belongs to the transition-metal ion, not the anion or the shiny metal — copper metal is reddish, but copper(II) sulfate solution is blue.
High melting point — except mercury
Transition elements have high melting points, with one exception: mercury is a liquid at room temperature. When you state this as a general property, add "except mercury" so a correct statement is not marked down — a general rule with a single named exception the examiner expects you to give.
Give contrasting pairs, not single facts
In a "differences from Group I" question each mark needs a contrasting pair, not a lone property: write "variable oxidation states, whereas Group I is always +1". Keep five ready — density, melting point, colour, oxidation states, catalysis.
Catalyst questions: name both parts
Name the catalyst AND its reaction — one mark for "iron", one for "Haber process". Learn the three pairs: iron with the Haber process, manganese(IV) oxide with hydrogen peroxide decomposition, nickel with hydrogenation of vegetable oils.
Oxidation state from a formula
If the name has a Roman numeral, that is the oxidation state — just add the sign. From a formula, balance the charges: fix known ions (O −2, Cl −1), make the total zero, solve for the metal, and report with the sign, e.g. +3 in .
Colour questions: be specific
Never answer a colour question with "coloured" — give the exact colour: copper(II) blue, iron(II) pale green, iron(III) orange-brown, potassium manganate(VII) purple. Check the Roman numeral, since it decides which iron colour is right.
All content in this topic is Extended (Supplement) — it is only examined on Extended papers.
Transition elements are the elements in the central d-block of the Periodic Table, found between Group II and Group III. In Cambridge IGCSE Chemistry the focus is on period 4 elements: iron (Fe), copper (Cu), manganese (Mn), nickel (Ni), zinc (Zn) and chromium (Cr).
Catalyst (mark-scheme definition): A substance that increases the rate of a chemical reaction and is chemically unchanged at the end of the reaction.
Oxidation state: A number (with sign) showing the apparent charge on an atom in a compound. The Roman numeral in a compound name states the oxidation state directly, e.g. iron(III) oxide means iron has oxidation state +3.
(Extended) Define the term "catalyst".
(Extended) Manganese(IV) oxide is used as a catalyst in the decomposition of hydrogen peroxide.
(a) Balance the equation for this reaction:
(b) State what happens to the manganese(IV) oxide during this reaction.
[3 marks]
